Implement Student CSV Bulk Import

[americac]

!!! Blocked By: #941 !!!

Description:
Add CSV bulk import functionality for Students with error reporting.

Tasks:

• Update Admin::StudentsController:
  • Add import action to handle CSV upload
  • Use BulkStudentImportService.import_from_csv
  • Validate file presence and format
  • Partition results (success/errors)
  • Build detailed error messages
  • Return results via flash messages
• Add template action to generate CSV template download
• Update index view:
  • Add import button to header
  • Create import modal with form
  • Show CSV requirements: "columns: classroom_id, username"
  • Add template download button
  • Implement JavaScript modal toggle

Acceptance Criteria:

• Import button opens modal
• Modal shows CSV requirements clearly
• Template download generates correct CSV format
• File upload accepts CSV files
• Import processes CSV using BulkStudentImportService
• Success count displayed after import
• Error messages display per-row details
• Modal closes after successful import
• Validation errors shown for missing file
• Flash messages styled correctly

Files to Update:

• app/views/admin/students/index.html.erb

Reference:

• app/controllers/admin/students_controller.rb (lines 17-40, 60-62)
• app/views/admin/students/index.html.erb (current - modal markup)
• app/services/bulk_student_import_service.rb